Space Elevator

[Laryssa]

Originally posted by Scarecrow:
As an aside, there was an old issue of White Dwarf (whilst it was still an RPG mag), that had a Traveller scenario that revolved around a heist job on a Beanstalk. Can't remember which issue now, but it had a beanstalk rooted on Mt Kilimanjaro with a huge city built around it. The stalk itself was a solid structure 10-20 metres across with two tunnels running through it, along which the shuttles (sort of vertical train carriages) ran between Earth and orbit. The job (and scenario) was set on the 16,000 km half way relay station; a small single deck structure some 40-50 metres square. I never ran it, but it looked like great fun. I believe it used Clarke's Fountain's of Paradise as inspiration, not sure about the science behind it.

Crow

One of the great fallacies of the Beanstalk idea is the idea that it needs a mountain to anchor it. The beanstalk doesn't even have to touch the ground, its own weight keeps it straight and taught, it doesn't need a mountain to do that.

In fact we could build a 12-hour beanstalk instead, one that orbits the earth over the Equator once every 12 hours instead of 24 in the direction of the Earth's rotation, that means that once every 24 hours, the Beanstalk will be over a given stop on the Earth. Its a rather simple matter to have an airplane fly up to it and unload its contents into an Elevator car and then fly down for a landing. The lower end of the elevater is high enough so that it always clears the highest mountain over the equator. This makes it rather convenient for traveller as they only have to travel to the equator and wait for the beanstalk rather than go to a specific spot on the Earth. As 12-hour orbit is lower than a 24 hour orbit, this reduces the engineering problems and the tensile strength requirements, and of course their is the dangling end that flys through the atmosphere. the tether can of course levetate off of the Earth's magnetic field and use this power to counter atmospheric drag.

 

[Scarecrow]

Originally posted by Andrew Boulton:
Ah, the classic "Tower Trouble". Issue 71.

Bloody hell, and I thought I was a mine of useless information!!!

 

[aramis]

Tom: that's just a variant of the Skyhook. The problem with that variant, however, is the EXCESSIVE drag it generates at 12 hour orbital speeds.

 

[parmasson]

Say is there a good place to build a ground anchored beanstalk?

I mean do you want it at the equator or south pole or . . .Scotland.

 

[Anthony]

Equator. Somewhere with minimal weather, lightning, wind, and earth movement. A ship floating in the pacific about a thousand miles from land has been suggested.

 

[Scarecrow]

I seem to recall that a floating base, not unlike a vast oilrig has been mooted. This means it can be moved out of the way of potential hazards should the need arise.

 

[Plankowner]

Good places for a Beanstalk are all probably within 5 degrees of the equator, much more than that and you get excessive lateral stress on the design, remember the "top" of the beanstalk is going to be in geostationary orbit, above the equator.

Borneo, Ceylon (Fountain's of Paradise), Central Africa and Northern South America are all likely candidates. You want to stay away from the locations on the Ring of Fire though, so eastern S.A. would be better French Gyana(sp?)or northern Brazil might work. Eastern Africa would also be a good candidate.

 

[Laryssa]

Originally posted by Aramis:
Tom: that's just a variant of the Skyhook. The problem with that variant, however, is the EXCESSIVE drag it generates at 12 hour orbital speeds.

Depends upon how high up the lower part is in the atmosphere. I'm thinking about the normal cruising altitude for jetliners. I think 1,000 miles per hour falls below the speed of sound. So a subsonic jet can dock with it while still flying and then unload its cargo and passengers. At the orbital altitude of the elevator their is a huge array of solar panels attached which runs a current down the length of the elevator. Atmospheric drag would tend to make the elevator lose orbital energy, but by running a current through the tether powered by the sun, you would make the tether act as part of a giant electric motor against the Earth's magnetic field, so long as you added electrical current, you can make up for the losses caused by atmospheric drag on the lower part of the tether.

 

[Uncle Bob]

Originally posted by Employee 2-4601:
I didn't say we couldn't get off earth - we can now, and with lift-assisted vehicles (for eaxmple X-33 if it wasn't cancelled )

A bit off-topic, but don't cry for Lockheed's X33, that dog never would have flown. Congress originally budgeted that billion dollars for the MDAC "Delta Clipper". NASA redirected the money into a technologically unsound "spaceplane" and Lockheed spent nearly the whole billion without finishing even a pretty mock-up. (Don't blame Lockheed. NASA put it out for bids and Lockheed futs a proposal out of 1960s concepts, knowing full-well that the Boeing/MDAC proposal was the good one. NASA fooled everyone and went with Lockheed.)

 

[Scarecrow]

That's a shame. I rather liked the X-33

 

[Plankowner]

The Delta Clipper was a COOL design. They even did a small prototype USING THEIR OWN MONEY and got it to fly, hover and land a couple of times. Saw the film a 10-12 years ago. I loved that design!

It looked like a large Dalek.

 

[Plankowner]

Laryssa, the sub-sonic jet could not catch a Skyhook dipping into the atmosphere. As I remember the designs, you needed a supersonic vessel to dock with it. Remember, it is travelling at several thousand miles an hour, not 1000 miles an hour.

Speed of sound is about 650 mph by the way, depending on atmospheric density.

 

[aramis]

at 8000 miles diameter, that's about 25000 miles circumference. Which puts the speed at about 2100 MPH, well above the speed of sound even at angels-50 (just shy of 10 miles up).

 

[Laryssa]

25,000 miles/24 hours = 1041.667 miles per hour.

The speed is relative to the surface of the Earth and the Earth is rotating once per 24 hours. The tangential velocity of the Earth's surface at its equator is 1041.667 miles per hour. The lower end of the skyhook is moving twice as fast as this, but the important velocity is the velocity relative to the spinning Earth's surface underneath is so you subtract the first velocity from the second to obtain 1041.667 miles per hour.

To get a relative velocity of 600 miles per hour the lower part of the skyhook would have to be traveling at a tangential velocity of 1641.667 miles per hour and 25,000 miles/1641.667 mph = 15 hours, so a 15 hour orbit will put the lower end of the skyhook at jetliner speed. Now building a space elevator with a 15 hour orbital period would be easier than one with a 24 hour period as their would be less stress on the tether, its center of mass would be in a lower orbit and the elevator itself would be shorter and you have the added convenience of a space elevator that revisits a given location on the equator about once every 42 hours. So you just wait at a certain spot and at a certain time you fly an airplane up to the tether.

 

[Plankowner]

But it would never be over the same part of the earth at the same time. Also, due to inclination, it would move north-south as well. Sure, all those things could be calculated and schedules built accordingly, but you would have to have several stations scattered around the globe and north and south of the equator, most of which are only used a couple of times a week, to make use of the skyhook.

 

[Laryssa]

It orbits along the plane of the Earth's rotation, not the plane of its orbit around the Sun. Because the gyroscopic effect affects both the orbit of the space elevator and the spin of the Earth, both keep to the same spin axis, this is know as angular momentum, it is why a top stays balanced on its point when spun.

 

[aramis]

Actually, Plankowner, one can set the orbit to be true equitorial...

The problem is the atmospheric drag.

The reason for spinning skyhooks as opposed to "flying bar" styles like Tom seems to be suggesting is to reduce and use the drag. You still need to keep these things boosted to keep them from falling out of the sky;

The tethered mass method is probably the best; it is held taught by its own velocity, and it's got relatively low (~0) wind speed to worry about. Plus, if damaged, only that below the break falls, and only if it's inside the equilibrium point.

 

[Uncle Bob]

Originally posted by Plankowner:
The Delta Clipper was a COOL design. They even did a small prototype USING THEIR OWN MONEY and got it to fly, hover and land a couple of times. Saw the film a 10-12 years ago. I loved that design!

It looked like a large Dalek.

Uh, not quite. The DC-X was funded by DOD (Balistic Missile Defence Office, although it was a notably lean, even "shoestring" project. When "Star Wars" ended Congress told NASA to take it over and they crashed it first flight. Congress then gave NASA a billion dollars to build the DC-Y and they blew it on the X33.

Just a note, what killed the X33 was the composite fuel tanks that never did work. OTOH, the DC-X had already flown with composite tanks.
If you like the DC-X, you will love these:
Douglas 1960s designs
Designing User-Friendly Civilian Spacecraft
Kankoh Maru
Roton
http://www.astronautix.com/craft/roton.htm

 

[Laryssa]

Originally posted by Aramis:
Actually, Plankowner, one can set the orbit to be true equitorial...

The problem is the atmospheric drag.

The reason for spinning skyhooks as opposed to "flying bar" styles like Tom seems to be suggesting is to reduce and use the drag. You still need to keep these things boosted to keep them from falling out of the sky;

The tethered mass method is probably the best; it is held taught by its own velocity, and it's got relatively low (~0) wind speed to worry about. Plus, if damaged, only that below the break falls, and only if it's inside the equilibrium point.

Spinning skyhooks are only accessible on certain points on the Earth's surface at certain times. Spaceships have to que up to wait for the end of the sky hook to drop down so they can be flung up into space. When an end isn't down, the skyhook isn't usable. However the moving space elevator is always accessible at some point on the Earth along the equator. Their is a certain limit to how much traffic it can handle at any given moment, but thickening the cable will allow it to handle more. Thickening the skyhook will allow it to fling a heavier single payload at one time, but since their are only a few points along the Earth's surface where they Skyhook touches down, this is bound to create congestion at these points as each payload waits its turn to be flung in space on a fixed schedule at fixed locations, whereas the non-spinning space elevator can pick up anyone along the equator. The elevator in part comes to them rather than they going to the Space Elevator downport.